Analysis Of Focal Effect Of Metasurfaces Based On Dielectric Nanobrick By Simulation

Metasurfaces are two-dimensional counterparts of metamaterials, and can be realized by employing nanoantennas array at the interface of two media. The conventional antennas can be referred to nanoantennas when its dimensions is enough small to react to optical wave and electromagnetic wave close to the infrared wave band. Therefore, the metasurfaces with various functions can be obtained by carefully designed the antennas array constructed the surface. The metasurfaces tune the phase of light without depending on the accumulation of phase at light propagation path. So the thickness of the surfaces can be much smaller than the wavelength of incident light. However, the inherent ohmic loss of metal restrict the efficiency of the light manipulation. The alternative approach is to employ dielectric materials instead of metal to produce nanoantennas. On the other hand, as people explore the microcosmos, small focal spot and high-resolution optical lens are indispensable. Lens with functionality to focus incident light beam to nanoscale region are requirement for far-field subwavelength microscopy imaging. However, due to the diffraction limit, the focus with Full Width at Half Maxima(FWHM) less than half the wavelength can not be obtained by conventional optical system in the far-field region.This article demonstrates a series of metasurfaces for subwavelength focusing. The metasurfaces are consist of three layers: the top layer is a array of silicon nanoantennas with high refractive index(nSi = 3.7). The middle layer is low refractive index(nspacer = 1.45) spacer layer. The bottom is silver reflective layer. The focsing efficiency of metasurfaces is much higher over 85% due to the reflective layer. We start this article with briefly review the development of the metasurfaces. Then we shortly introduce the FDTD method(Finite Difference Time Domain, FDTD) used in simulation. The results of text are calculated by commercial software FDTD Solutions(Lumerical Solutions Inc., Vancouver, Canada). Then, this article analyze the local surface plasmon resonances(LSPRs) in metal and electric and magnetic dipole resonances in dielectric antenna. It turned out that the two resonances can tune the phase and amplitude of scatter wave. Then, phase continuity is breaked at the location of the antennas. The beam transmission follow generalized Snell's law when phase discontinuity arise at the interface. To illustrate the high efficiency of the metasurfaces, the multiple reflections model, which is like Fabry-Pérot cavity, is employed in the text. The results indicate that ultra-high reflectivity(97%) and broad spectral response(800 nm-1200 nm) of metasurfaces can be obtained with optimizing the thickness of the top silicon nanoblocks and the spacer layer. Finally, in order to realize beam deflection and focusing, the desired phase distribution is provided by arrays consisted of antennas selected from the scan results, and the phase distribution of metasurfaces are obtained by calculation. The reflection or transmission of the deflecting metasurfaces follow the generalized Snell's law with changing the incident angle. For focusing metasurfaces, fixed focal length as 10 ?m, the wavelength of the incident wave as 1000 nm, the FWHM of focal spot can be less than half of the incident wavelength(440 nm). The influence of numerical aperture(NA), incident wavelength, and angle of incidence on the effect of focusing metasurfaes are analyzed.